Moving target indicator radars have been used to detect and track targets since World War II. In searching for targets, a narrow beam is typically scanned through a large area of interest. Radar searches in years after WWII were mechanically performed by rotating an antenna on a gimbal. For more recently developed systems, the beam is scanned electronically with a phased array antenna using switches, ferroelectric, or other active devices. Phased array antennas can be expensive and consume large amounts of power. Another less expensive approach for scanning a radar beam is to use a frequency scanning antenna. In systems using frequency scanning antennas, there is a trade-off between generally good angular resolution in exchange for poor range resolution. Frequency scanning antennas, however, do not typically support 2-D scanning and high range resolution system requirements. Consequently, radars using frequency scanning antennas have poor range resolution but good angular resolution.
Numerous techniques have been developed to reduce the cost, size, and power requirements of electronically scanning antennas (ESA). One technique to reduce the cost is to aperiodically place a smaller number of antenna elements across an aperture as described in more detail in J. T. Bernhard, et al, “Wideband random phased arrays: theory and design”, Wideband and Multi-band Antennas and Arrays, 2005, hereby incorporated by reference. This technique that has been shown to be feasible based upon performance metrics such as peak sidelobe level as described in Y. T. Lo, “A mathematical theory of antenna arrays with randomly spaced elements,” IEEE Trans. on Antennas and Prot., vol. 12, pp. 257-268, March 1964, hereby incorporated by reference (hereinafter “Lo Mathematical Theory Article”). The “Lo Mathematical Theory Article,” explores the possibility of a large antenna array with randomly spaced elements and finds the required number of elements is closely related to the desired sidelobe level and is almost independent of the aperture dimension, the resolution (or the beam width) depends mainly on the aperture dimension, and the directive gain is proportional to the number of elements used if the average spacing is large. The “Lo Mathematical Theory Article” points to then recent advances in space exploration as having shown a great need for antennas with high resolution, high gain and low sidelobe level. Although the “Lo Mathematical Theory Article” touches upon the use of “high speed computers,” the focus is on a technique for optimizing the design of phased arrays.
In the radar system disclosed in Statutory Invention Registration No. H1773, entitled Ultra-wideband Active Electronically Scanned Antenna, the elements are spaced by λ/2 and the maximum delay corresponds to 360° of phaseshift. The various lines feeding the elements of the array at the aperture have bias delays that make them all equal in length, thereby giving a broadside beam.